Eager load associations in admin and API collection endpoints

[ikraamg]

A few admin and API list endpoints render per-row associations that were not eager loaded, so the query count grew with the number of rows. This eager loads what each view already renders.

• solidus_admin products index: the stock column calls total_on_hand, which loaded each variant's stock items and their stock location per row
• solidus_admin stock items index: each row loaded its variant, the variant's product, images, option values, and the stock location
• API stock movements index: each movement loaded its stock item and the variant data the listing renders (product, prices, stock, images, option values)
• API users index: each user loaded its billing and shipping address
• admin order payments list: each payment loaded its payment method

Each fix is a controller-level includes. The specs assert the relevant table is loaded in a single batched query as the number of rows grows.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (8d781ac) to head (342a3dc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6504   +/-   ##
=======================================
  Coverage   89.68%   89.69%           
=======================================
  Files         993      993           
  Lines       20863    20864    +1     
=======================================
+ Hits        18712    18713    +1     
  Misses       2151     2151           

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